An example of a synchronous machine with which the present invention may be used is disclosed in U.S. Pat. No. 3,210,644. Such a machine has exciter and alternator portions. Moreover, the machine includes relatively stationary and relatively movable portions usually referred to as exciter and alternator stators and exciter and alternator rotors. An AC signal is induced in the exciter rotor. The rotor also carries a rectifier, preferably a full wave rectifier, which rectifies the AC signal from the exciter and provides the same as a rectified signal input to the alternator rotor, which often also is referred to as the alternator field or alternator field winding. The rectified signal in the rotating field induces in the alternator stator (armature) an AC signal which typically may be used as the machine output and also may be used as an input to the exciter stator.
In such machines, especially those of the type that are brushless synchronous machines, it is desirable to minimize the number, size, and weight of the components included in the rotor while maximizing reliable operation of the machine.
Under certain conditions, an alternating or AC voltage may be induced in the field winding of a synchronous machine. Two common conditions causing such induced voltage are (1) when the machine is started as an induction motor and (2) when the machine is parallelled or synchronized with another power source. Due to a high turns ratio between the armature and field windings, the induced field voltage may be very high, for example several thousand volts or more. Since the induced voltage is an AC voltage, the diodes of the field rectifier, e.g. the full wave rectifier mentioned above, will be reverse biased during one-half of each cycle of the induced AC voltage. If the magnitude of such induced voltage exceeds the peak inverse voltage (PIV) rating of the diodes, the diodes could be damaged.
It is desirable to avoid such damage due to peak inverse voltage to the diodes or other rectifier means in the rotor of a brushless synchronous machine and in other devices as well; and the present invention is directed to an arrangement to provide such overvoltage protection.